Fire-generated mosaics of habitat legacies such as dead and dying trees are key structural components in boreal forest ecosystems that support diverse saproxylic beetles. Our study sought to elucidate the spatial pattern of community composition (beta diversity) of saproxylic beetles along gradients of post-fire habitat legacies (tree species composition, tree size [dbh], and burn severities) and geographical distance. For 2 y, we reared saproxylic beetles from 360 logs retrieved from 72 sites in burned forests. Tests were performed to explain the overall beta diversity (βsor) by partitioning it into its 2 components: the “species spatial turnover” due to species replacement (βsim) and the “richness-driven” beta diversity due to species richness differences (βrich)- Variations in tree species, tree size, and burn severity had significant effects on overall beta diversity (βsor) of saproxylic beetles; these effects varied according to the differential influence of these factors on the 2 distinct components of beta diversity. Tree composition had notable effects on species spatial turnover (βsim), for which saproxylic species composition differed between jack pine and black spruce, and it was more variable between jack pine sites than between black spruce sites. On the other hand, variation in tree size was primarily responsible for the richness-driven beta diversity component (βrich), which was highest between the smallest and largest dbh groups and lowest between largest and mid-sized trees. Similarly, richness-driven composition differed significantly across severity gradients and was highest across low to high severity and lowest between low-severity stands. Broader geographical distance per se did not affect compositional patterns of saproxylic beetles, yet the landscape context could have some effect. These results could have crucial implications for post-fire management, which also aims to efficiently conserve saproxylic beetles. The significant spatial turnover in saproxylic composition between black spruce and jack pine and the underlying host-tree specificity suggest that a mosaic of both tree species should be maintained in the landscape. The richness-driven beta diversity pattern along the tree size and severity gradients implies that it may be necessary to prioritize the most species-rich classes, such as larger trees with lower severity burns, but with qualifications to cater also for species displaying idiosyncratic distributions.